Hydrophobic Interaction of the Water-Soluble ABA-Type Block Copolymer Composed of 2-Hydroxyethyl Methacrylate and Ethylene Oxide and Its Conformational Transition

Abstract

The water-soluble ABA-type block copolymer composed of a hydrophobic poly(2-hydroxyethyl methacrylate) and a hydrophilic poly(ethylene oxide) was synthesized. The correlation between the structure of the block copolymer in dilute aqueous solution and its hydrophobic interaction was studied in comparison with random copolymers whose hydrophobic groups are statistically distributed along the chain. Fluorometric measurements using ANS as a probe for hydrophobic region were carried out on the aqueous solution of the block copolymer in the monomolecular state. The hydrophobic domain structure of the monomolecular block copolymer is discussed with respect to the change in the thermodynamic parameters for the binding process of ANS to the polymer chain. The binding ability of the block copolymer for an ANS molecule was found to be larger than that of corresponding random copolymers. This indicates that the mode of the arrangement of hydrophobic groups along the chain affects the hydrophobic interaction and that the block copolymer forms the large hydrophobic region which results from the aggregation of these hydrophobic blocks. It was observed that the temperature dependence of the binding constant of the block copolymer displayed a sharp transition within a narrow temperature range, indicating that the structural change takes place within a molecule. The thermodynamic parameters for both states of the block copolymer below and above the transition temperature were determined independently from the temperature dependence of the binding constant. This means that the hydrophobic domain of the block copolymer becomes larger in the lower temperature range than in the higher, presumably because of the segregation between the hydrophobic and the hydrophilic blocks.